For the growing demands of bandwidth in 4G LTE or upcoming 5G mobile network, a basic consensus on cloud Coordinated Radio Access Network (C-RAN) that dominates the Next Generation Mobile Network (NGMN) has been achieved among the organizations and leading companies in the industry/market. To strongly support and facilitate the implementation of C-RAN, key technique breakthroughs in massive-capacity fronthaul transmission will become powerful drives from the aspects of both technology and the business.
Recently, standardization groups such as Full Service Access Network (FSAN) and International Telecom Union-Telecommunication (ITU-T) forum have started working groups on Mobile FrontHaul (MFH) transmission issues. Meanwhile, China Mobile, the world's largest communication corporation, has been spearheading trials and plans to deploy C-RAN systems as early as 2015. Japan's NTT DOCOMO said it will follow the deployment of C-RAN systems in 2016. The highlight of C-RAN technology is that it can carry out the centralized baseband Digital Signal Processing (DSP) calculation, and efficiently control/manage the cost for the Baseband Unit (BBU) processing, concentrated cooling provision and real estate purchasing/renting.
In the existing technology, there are two types of transmission approaches to implement MFH of C-RAN: Digitized Radio over Fiber (D-RoF) and Analog Radio over Fiber (A-RoF). The two most typical protocol implementations of D-RoF are Common Public Radio Interface (CPRI) and Open Base station Standard Initiative (OBSAI) as shown in FIG. 1B. FIG. 1A shows a schematic diagram of a D-RoF based optical network architecture. Since the degradation induced by the D-RoF based MFH transmission is negligible, D-RoF is preferred in current 4G era, but D-RoF requires very large bandwidth resources. With the booming of MFH capacity enhancement such as massive MIMO advanced applications, D-RoF will become a suboptimal selection due to its requirement on bandwidth.
A second MFH approach is A-RoF; with the assistance of advanced DSP, the approach can sustain the signal quality in the MFH operation, exhibit flexibility in terms of wireless parameters and require much less optical transmission bandwidth compared with D-RoF. In the network architecture as shown in FIG. 2A, the data to (or from) each antenna is carried over individual Intermediate Frequency (IF) sub-bands in a Quadrature Amplitude Modulation (QAM) format. The conceptual diagram corresponding to FIG. 1B is shown in FIG. 2B. It indicates that by introducing a Digital to Analog Converter (DAC) and an Analog to Digital Converter (ADC) respectively in the BBU and a Remote Radio Header (RRH), CPRI data can be mapped to QAM constellations in an analog manner. Due to the maturity of DSP technology, the analog transmission can deliver information with high spectra efficiency at affordable expenses. For instance, a cell (e.g., equipped with 24 antennas) requires only a 480 MHz bandwidth in theory for 4G LTE MFH, which means that one 1 GHz optical transceiver (TRx) is sufficient to accommodate 6 RRHs (corresponding to 6 sectors) with 48 antennas, while one 10 GHz D-RoF TRx can only transmit 8 antennas. A brief comparison between the two approaches, i.e. A-RoF and D-RoF in terms of performance has been concluded in Table 1.
TABLE 1Comparison between D-RoF and A-RoF MFH8 Antennas3 SectorsRequestProtocolFormatNoteCPRI MFHthree 10G TRxTDMOOKTRxA-RoF MFHone 1G TRxIFDMOFDM-QAMADC/DAC
Currently, one of the most challenging problems to block A-RoF MFH from being widely implemented is the stringent request for the broadband ADC/DAC module. As shown in FIG. 2A, in an A-RoF MFH based on the C-RAN architecture, data for each antenna is carried on individual IF sub-carriers in the MFH link, some of which (e.g., IF sub-bands from #1 to #8) are allocated at low frequency bands and their corresponding IF bandwidths locate at 50 MHz to 210 MHz, and some of which (e.g., IF sub-bands from #17 to #24) are allocated at very high frequency bands and their corresponding IF bandwidths locate at 370 MHz to 530 MHz. Therefore, the broadband ADC and DAC must be installed in the RRH, where its target data is allocated at the high end of the frequency axis. In fact, IF sub-band allocation algorithms can be determined flexibly in the BBU, which means that each RRH must be equipped with a full-bandwidth ADC and DAC. In a word, with the increase of antenna numbers in the MFH links, the broadband ADC and DAC are required in the BBU and RRH, which requires a very high cost.
Currently, there is no solution to resolve this problem yet. However, one of the most simple and straightforward approaches for IF multiplexing/de-multiplexing is to employ a HardWare (HW) frequency mixer. By using the HW frequency mixer, target data can be converted between the baseband and IF channels and a narrow-band DAC or ADC are competent to process the data on each channel. However, the problem is that it would ask for as many HW frequency mixers as the antennas in one cell (e.g., 24 antennas or more in NGMN), and the product cost as well as complexity issues make this pure-HW depended approach not practical.